A few years ago Robert Rector and Kirk Johnson of the Heritage Foundation did an analysis of the 1995 National Survey of Family Growth and found that for women 30 or older those who were monogamous (only one sexual partner in a lifetime) were by far most likely to be still in a stable relationship (80 percent). Sleeping with just one extra partner dropped that probability to 54 percent. Two extra partners brought it down to 44 percent. Who would have thought that the price of sleeping with even one partner would lead to divorce for almost half of those who had only one extra tryst?

The problem here is blatant confusion of correlation with causation. The fact that women who’ve had fewer sexual partners are more often in stable marriages does not mean that having fewer sexual partners causes more stable marriages. It just means the two phenomena happen to occur together. Julian provides one possible reason: the correlation is driven by a single underlying factor, possession of conservative values. I feel sure that’s part of the story. But isn’t there an even simpler explanation?

Let A = fewer sexual partners, B = more stable relationships. Fagan says A leads to B. Julian says another factor C (conservative values) could lead to both A and B. I suggest that we might actually have reverse causation: B leads to A. People who happen to find stable relationships early in life don’t have to shop around as long, and therefore accumulate fewer sexual partners. If you marry the first person you ever have sex with, and you stay together, then (barring infidelity) you’ll only have one partner your entire life. If you don’t meet your soul mate until the age of 40, on the other hand, then you’re likely to rack up a few more partners in the meantime. When you get married, your number stops going up (again barring infidelity); when you divorce and go back on the market, your number starts to rise again. So if you take a snapshot at any moment in time, those currently in stable marriages will tend to have had fewer partners, and those on the market will tend to have had more partners. This creates an immediate correlation between the two phenomena, without any necessary role for conservative values (Julian’s explanation) or destabilizing effects of promiscuity (Fagan’s).

[Addendum: On rereading Julian's post, I think he may have implied my point here by saying those conservative values can cause people to "lock in early." My point is that even without conservative values, you're going to get a correlation because of the very nature of searching for a mate. Those who lock in early, for whatever reason, will tend to have fewer partners; those who take longer will tend to have more.]